In the electromagnetic spectrum, terahertz radiation is located between microwaves and the infrared or visible optical radiation, respectively. Although the application of terahertz radiation for the time domain spectroscopy, TDS, was already described in the article “far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors” of D. Grischkowsky in J. Opt. Soc. Am. B/Vol. 7, No. 10/October 1990, terahertz waves were hardly used because the generation of terahertz radiation was technologically very sophisticated until recently. A breakthrough for the terahertz technology came with the insight that terahertz radiation could be generated by irradiating ultrashort laser pulses (i.e. laser pulses with a duration of less than 10 picoseconds) onto a suitable non-linear material or into a photoconductive semiconductor element, i.e. between both electrodes of a dipole antenna provided on the semiconductor material. The latter is described e.g. in U.S. Pat. No. 5,729,017 A. Basics for the generation and application of terahertz radiation are described in the book “Terahertz sensing technology, volume 1: Electronic devices and advanced systems technology,” D. L. Woolard et al., World Scientific Publishing Co. Pte. Ltd. 2003.
Meanwhile, the most important areas of application for terahertz radiation are imaging methods—as described in U.S. Pat. No. 5,710,430 A—and spectroscopy methods, as described e.g. in U.S. Pat. No. 5,789,750. The advantage of terahertz radiation in comparison to other electromagnetic radiation, e.g. x-ray radiation, is that the absorption spectra of several materials are highly modulated in the terahertz range, and have a very characteristic course. Further, terahertz waves penetrate most non-metallic objects such as paper, cardboard, plastics and some semiconductor materials with hardly any attenuation. For these reasons, terahertz radiation is particularly suited for non-destructive methods of testing, or for the detection of certain gases or moisture.
The broader the field of potential applications, the more interesting it is to be able to generate terahertz radiation with low maintenance requirements, at low cost and in spatially small units. A device developed in this way for generating or receiving electromagnetic radiation in the terahertz range is described in EP 1 230 578 B1. In this device, a coupling end of a wave guide is guided into a housing. A comparatively large relay optic focuses the ultrashort light pulses exiting from the wave guide fiber onto a terahertz converter. Like in the present invention, the terahertz converter may e.g. be a photoconductive element according to U.S. Pat. Nos. 5,729,017, 5,420,595, 5,663,669, Applied Physics Letters 45, p. 284, 1984, Applied Physics Letters 55, p. 337, 1989, or an electro-optic or magneto-optic device according to U.S. Pat. Nos. 5,952,818 or 6,111,416.
If the converter is a photoconductive element, then an electrically conductive dipole antenna is present in or on a semiconductor material, both poles of which are arranged at a mutual distance of merely several micrometers. The ultrashort laser pulses are focused by the relay optics between the two electrodes, in order to instantaneously release free electrous. If a voltage is applied to both electrodes of the dipole antenna, this leads in compliance with the Maxwell equations to an instantaneous flow of current, and hence, to the emission of terahertz radiation. In this case, the dipole antenna is used as an emitter. If no voltage is applied, the free electrons generated at the dipole antenna may be used for the detection of incoming terahertz radiation. In this case, the antenna operates as a receiver for the terahertz radiation. EP 1 230 578 B1 suggest to provide a comparatively compact module by arranging the terahertz converter, the relay optics and the coupling end of the wave guide in a common housing. Although EP 1 230 578 B1 gives first hints towards an industrially applicable terahertz source, there is still a potential for improvement.
WO 2007/143542 A2 discloses the frequency doubling of femto-second pulses of an Erbium doped fiber laser, in order to generate terahertz radiation with the frequency doubled pulses.
US 2005/0100866 discloses a terahertz emitter, which may be introduced e.g. into the human body in the form of a probe.
U.S. Pat. No. 6,014,249 A is directed to the temperature dependency of the frequency doubling of ultrashort laser pulses. This document discloses a heating in order to control the temperature of a frequency doubling crystal and, thus, the wavelength of the emitted light.
WO 2007/082371 A1 describes the application of polarized radiation, including polarized terahertz radiation, for measuring the orientation of fibers in materials such as wood or paper.
However, conventional terahertz sources exhibit potential for improvement in several aspects, in particular, with respect to their manageability.
Hence, it is the object of the present invention to provide, with as simple means as possible, a device for generating or receiving terahertz radiation, which is further optimized with respect to a compact structure, a reliable, low maintenance operation, and with respect to its optical efficiency.